I have installed quite a few bone nuts and saddles over the years on various instruments. I got some bone from one source (can't even remember for sure who it was now) that did seem as soft as some of the plastic or plastic-like nuts and saddles. It dented just like Tusq (I like Tusq, but it dents at the saddle and is nowhere near as durable as hard bone). It was definitely bone though, as it had the distinct smell while sanding it.
That said, not all bone is the same, that's for sure.
Just an example: Stewmac.com has two different types of bone. One is unbleached and the other bleached white. The unbleached is harder than the bleached white. Their information says that, and I've tried both and agree. Even their unbleached white is harder than the bone I mentioned earlier that dented easily.
It's possible it's just a softer bone on your uke. Of course it's possible it is plastic too, I just thought I'd throw this out there. Not everyone know this, and I wouldn't know myself had I not had an experience with the softer bone.